This invention relates to personal electronic devices, such as wristwatches, pocket pagers, calculators, and organizers, having an EL lamp and a piezoelectric buzzer powered from a single inductor.
An EL lamp is essentially a capacitor having a dielectric layer including a phosphor powder which glows in the presence of a strong electric field and a very low current. The dielectric layer is held between two electrodes, one of which is transparent. Because the EL lamp is a capacitor, an alternating current (AC) must be applied to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage and the current through the EL lamp ceases.
For personal electronic devices such as wristwatches, pocket pagers, and cellular telephones, an EL lamp is driven by an inverter which converts direct current from a small battery into alternating current. In order for an EL lamp to glow sufficiently, a peak to peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder.
While there are many ways to increase voltage, e.g. by using a transformer or a voltage doubler, most applications for an EL lamp use what is known as a xe2x80x9cflybackxe2x80x9d inverter in which the energy stored in an inductor is supplied to the EL lamp as a small pulse of current at high voltage. The inverter typically operates at high frequency (4 khz. or more) to minimize the size of the magnetics, i.e. the inductor or transformer, in the inverter.
FIG. 1 is based upon the disclosure of U.S. Pat. No. 4,527,096 (Kindlmann). When transistor 14 turns on, current flows through inductor 15, storing energy in the magnetic field generated by the inductor. When transistor 14 shuts off, the magnetic field collapses at a rate determined by the turn-off characteristics of transistor 14. The voltage across inductor 15 is proportional to the rate at which the field collapses (xcex4i/xcex4t). Thus, a low voltage and large current is converted into a high voltage at a small current.
The current pulses are coupled through diode 16 to the DC diagonal of a switching bridge having EL lamp 12 connected across the AC diagonal. Assuming that transistors 18 and 19 are conducting, the same amount of energy is supplied to lamp 12 each time transistor 14 turns off and, therefore, the voltage on the lamp is pumped up by a series of current pulses from inductor 15 as transistor 14 repeatedly turns on and off. Diode 16 prevents lamp 12 from discharging through transistor 14. If transistor 14 were switched on and off continuously, the pulses would charge lamp 12 to the maximum voltage available from inductor 15, e.g. about 140 volts. Since an EL lamp needs an alternating current or a variable direct current, the lamp would glow initially and then extinguish when the capacitance of the lamp became fully charged.
To avoid this problem, the transistors in opposite sides of the bridge alternately conduct to reverse the connections to lamp 12. The bridge transistors switch at a lower frequency than the frequency at which transistor 14 switches. The four bridge transistors are high voltage components, adding considerably to the size and cost of the circuit. In addition, the circuit is not single ended, i.e. one cannot ground one side of lamp 12, which is preferred.
One could use separate inverters for driving an EL lamp and a buzzer. In many applications, particularly watches, a second inverter is difficult to add, primarily because of the cost of a second inductor. It is known in the art to power a piezoelectric buzzer and an EL lamp from a single flyback inverter. FIG. 2 is based upon the disclosure of U.S. Pat. No. 4,529,322 (Ueda). In inverter 20, transistor 14 is switched on and off at about 8 khz. to charge lamp 12. When transistor 21 is conducting, lamp 12 is discharged.
There is an average DC bias across lamp 12, approximately equal to one half the maximum voltage, because the lamp is charged in only one direction and then discharged. DC bias on an EL lamp can cause corrosion and shorting of the electrodes of the lamp, particularly at elevated temperature and humidity, decreasing the life of the lamp.
Another problem with inverter 20 is that transistors 21 and 22 draw current from terminal 13 through inductor 15. This current is wasted since it does not contribute to powering lamp 12, thereby reducing the efficiency of the inverter and decreasing battery life.
A third problem with inverter 20 is that switch 25 is necessary for isolating piezoelectric buzzer 26 from the high voltage pulses applied to lamp 12. High voltage pulses stress the piezoelectric element and can cause failure. In the Ueda patent, switch 25 is one of two ganged switches actuated by undisclosed means.
FIG. 3 is based upon the disclosure of U.S. Pat. No. 5,313,117 (Kimball). Inverter 30 includes transistor 31, inductor 32, and transistor 33 connected in series between voltage source 34 and ground. Inductor 32 is alternately connected through transistors 35 and 37 to lamp 27. Diode 36 is connected in a series with transistor 35 for preventing the transistor from operating in the inverse active mode, i.e. preventing transistor 35 from conducting current from the ground terminal through the forward bias based-collector junction when the voltage on lamp 27 is negative. Similarly, diode 38 prevents transistor 37 from operating in the inverse active mode when the voltage on lamp 27 is positive and greater than the battery voltage. The transistors, resistors, and diodes are implemented on a single chip. The inductor and capacitors are external devices coupled to the chip on a printed circuit board.
External logic circuitry provides a series of pulse bursts alternately on output lines xe2x80x9cXxe2x80x9d and xe2x80x9cYxe2x80x9d. These bursts are coupled to the bases of transistors 31 and 33 and cause the transistors to conduct alternately, thereby providing positive and negative half wave voltages to lamp 27. Inverter 30 produces alternating current at a single ended output and one side of lamp 27 can be grounded.
It remains a problem in the art to drive an EL lamp and a buzzer from a single inverter with as few components as possible with no waste current and with no DC bias.
In view of the foregoing, it is therefore an object of the invention to provide a personal electronic device with an inverter having a single inductor for providing alternating current to an EL lamp and direct current pulses to a buzzer.
Another object of the invention is to provide a personal electronic device with an inverter having no waste current.
Another object of the invention is to provide a personal electronic device with an inverter having no DC bias.
The foregoing objects are achieved in this invention in which a personal electronic device includes an inverter having an output coupled to the junction of an EL lamp and a buzzer. The lamp and the buzzer are coupled in parallel with each other to ground or are coupled in series between a DC supply and ground.